1. Field of the Invention
The present invention relates to an aqueous preparation for the pickling and activation of aluminum-steel composites prior to electroless dip tinning. Examples of aluminum-steel composites are sliding bearings, bushes, wear disks, dry sliding bearings, etc., for pumps, motors and gear boxes. After pickling and activation, uniform, particularly strongly adhering tin coatings are obtained on the aluminum and steel surfaces of the substrate in a subsequent dip tinning process.
2. Prior Art
EP-A-0 278 752 discloses the tinning of substrates of aluminum alloys by an exchange method using acidic tin salt electrolytes after a pretreatment comprising degreasing and pickling.
Prior art systems comprising degreasing, pickling and tinning give unsatisfactory results when employed on substrates comprising aluminum-steel composites:
the cleaning, pickling and tinning of the steel surfaces and the aluminum surfaces is non-uniform, PA1 the deposition of tin on the steel surfaces occurs non-uniformly and does not give a closed surface, PA1 the adhesion of the tin layer deposited on the aluminum is unsatisfactory. PA1 sulfuric acid for pickling steel surfaces, PA1 hexafluorosilicic acid for pickling aluminum surfaces, PA1 surfactants for uniformly wetting the substrate surfaces with the solution, PA1 transition metal cations for optimizing the pickling and activation of the aluminum surface, PA1 nitrate and/or nitrite ions for optimizing the pickling and activation of the aluminum surface.